Obviously the Dollars Trilogy is key (The Good, The Bad and The Ugly, A Fistful of Dollars, For a Few Dollars More). I haven't seen them personally, but I'm guessing any of the original Django films would be a good bet as well - the 1966 version is apparently the first and one of Tarantino's favorites (hence the homage).

Once Upon a Time in the West is another classic - but I will leave you with a link - if you really want to watch the best spaghetti westerns before seeing Django Unchained, you may as well take advice from Tarantino himself: